This invention relates generally to microelectromechanical system (MEMS) devices. More particularly, it relates to actuating and measuring the motion of a micro-machined electrostatic actuator.
Prior methods for capacitive position sensing of MEMS devices have been focused towards inertial sensors such as accelerometers and gyroscopes. These earlier techniques were subject to the following disadvantages:
1. Sensitivity to low-frequency amplifier noise, such as voltage offset and 1/f noise.
2. Sensitivity to capacitive xe2x80x9cfeed-throughxe2x80x9d of the drive signal into the measurement signal. Many prior art MEMS drive systems apply a small AC signal on top of a DC drive voltage. The AC signal is used to measure the capacitance of the MEMS device. Variations of the DC signal can be picked up by the capacitance sensing circuitry. Such pick-up is known as feed-through and is a form of noise. This noise may be significant since prior art capacitive sensing systems provide only a small amount of current for capacitive sensing.
3. Non-linear actuation force.
4. Analog interface. An analog interface adds to the complexity of the control circuitry.
The disadvantages associated with the prior art are overcome by embodiments of the present invention directed to methods and apparatus for varying and measuring the position of a micromachined electrostatic actuator using a pulse width modulated (PWM) pulse train. According to a method for varying the position of the actuator, one or more voltage pulses are applied to the actuator. In each of the pulses, a voltage changes from a first state to a second state and remains in the second state for a time xcex94tpulse before returning to the first state. The position of the actuator may be varied by varying the time xcex94tpulse. A position of the actuator may be determined by measuring a capacitance of the actuator when the voltage changes state, whether the time t is varied or not.
An apparatus for varying the position of a MEMS device may include a pulse width modulation generator coupled to the MEMS device an integrator coupled to the MEMS device and an analog-to-digital converter coupled to the integrator. The integrator may measure a charge transferred during a transition of a pulse from the pulse generator. The integrator may comprise an amplifier, an integrator capacitor, a hold capacitor, a compensation voltage generator and three switches. The hold capacitor and integrator capacitor may be coupled to a MEMS device. The integrator capacitor, hold capacitor, and compensation voltage generator may be selectively coupled to the amplifier by two of the switches. The MEMS device and hold capacitor may be selectively coupled to ground by a third switch.
Embodiments of the present invention that use a switching integration technique are relatively insensitive to noise sources that have been problematic in the prior art.
Embodiments of the present invention use time-multiplexing to separate the measurement period from the driving period, eliminating cross-talk between the drive and measurement signals.
Because embodiments of the present invention use a constant amplitude PWM pulse train, they are not subject to the quadratic voltage to force non-linearity found in typical electrostatic actuation techniques.
Embodiments of the present invention use an entirely digital interface, rendering them compatible with modern digital feedback control systems.